The present invention relates to a method of making a call in an electronic private branch exchange system (hereinafter referred to as EPBX), more particularly to a method of calling a subscriber pre-engaged in a call and thereby performing call-wait and call-alternation.
In a conventional EPBX, when making a call to a subscriber pre-engaged in a call, a calling party has to either try again or make a call reservation to again call the subscriber only after he ends his own call engaged. In addition, when switching an external call from a central office line (hereinafter, referred to a "C.O. line") to an internal subscriber of the EPBX, it is not possible to give notice to the subscriber of the arrival of the call if the subscriber is already engaged in other call. As a result, the conventional EPBX has such a drawback that the calling subscriber of the C.O. line has to give up the call and try it again later on, and furthermore, even if it is possible to notice the desired internal subscriber of the arrival of the call from a C.O. line subscriber, the only way the called party can talk with the calling party is by quitting the current telephone conversation.